Use Java 8 as Target for Gradle Example (#339)

As discussed in gradle/gradle#33964 and apache/logging-log4j2#3754, Gradle fails the build if a `compileOnly` dependency is not compatible with the Java version specified via `options.release`, even though such dependencies don’t impact runtime behavior.

This PR updates the Gradle example to explicitly target **Java 8**, allowing us to surface and detect such compatibility issues through our regression tests. This helps ensure our published artifacts remain usable in Gradle builds targeting Java 8, and catches potential issues earlier in our release cycle.

Author: ppkarwasz

log4j-samples-gradle-metadata/build.gradle

@@ -30,6 +30,10 @@ dependencies {
 }
 
 tasks.withType(JavaCompile).configureEach {
+    // Test compatibility of `compileOnly` dependencies with Java 8
+    //
+    // See: https://github.com/gradle/gradle/issues/33964
+    options.release = 8
     options.compilerArgs.add("-Werror") // treat all warnings as errors
     options.compilerArgs.add("-Xlint:all") // includes 'classfile' check
 }
@@ -55,14 +59,14 @@ tasks.register("assertRuntimeClasspath") {
     inputs.files(configurations.runtimeClasspath)
     doLast {
         def actual = inputs.files.collect { it.name.replaceAll("-[0-9].*", "") }
-        assert actual.sort() == expectedRuntimeClasspath : "Unexpected runtime classpath: $actual"
+        assert actual.sort() == expectedRuntimeClasspath : "Unexpected runtime classpath:\nActual: $actual\nExpected: $expectedRuntimeClasspath"
     }
 }
 tasks.register("assertCompileClasspath") {
     inputs.files(configurations.compileClasspath)
     doLast {
         def actual = inputs.files.collect { it.name.replaceAll("-[0-9].*", "") }
-        assert actual.sort() == expectedCompileClasspath : "Unexpected compile classpath: $actual"
+        assert actual.sort() == expectedCompileClasspath : "Unexpected compile classpath:\nActual: $actual\nExpected: $expectedCompileClasspath"
     }
 }
 

log4j-samples-gradle-metadata/src/main/java/module-info.java

@@ -24,6 +24,6 @@ public class App {
     org.apache.logging.log4j.spi.LoggerRegistry<?> r; // needs jspecify
 
     public static void main(String[] args) {
-        System.out.println("Hello from: " + App.class.getModule().getName());
+        System.out.println("Hello from: " + App.class.getName());
     }
 }